(i) Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more particularly, it relates to a method for forming a silicide film of a high-melting metal in self alignment on a diffusion layer or a gate electrode of a MOS transistor.
(ii) Description of the Related Art
The miniaturization and the high densification of semiconductor elements have energetically been advanced, and nowadays, super-highly integrated semiconductor devices such as memory devices and logic devices designed at a size level of 0.15 to 0.25 .mu.m have been made. With the high integration of such semiconductor devices, it has gotten more important more and more to decrease the width of gate electrodes and the width of diffusion layers and to reduce the thickness of materials constituting the semiconductor elements. However, the decrease of the width of the gate electrodes and gate electrode wires and the reduction of the thickness of gate electrode materials inevitably give rise to the increase of wiring resistance and have a large influence on the circuit delay. For the solution of these problems, it is an essential technique to lower the resistance of the gate electrodes by the utilization of a high-melting metal silicide. In particular, a self-align-silicide formation technique using titanium metal as the high-melting metal has become an extremely important technique for the miniaturization of MOS transistors.
A first conventional manufacturing method of the MOS transistor having a self-aligned-silicide structure will be described with reference to FIGS. 5 and 6. In the first place, as shown in FIG. 5(a), an element separation insulating film 502 is formed in a predetermined area on a silicon substrate 501 by a known LOCOS method. Next, an ion implantation of an impurity for a channel stopper is conducted, and a gate insulating film 503 is then formed by a thermal oxidation method. Next, a polysilicon film having a thickness of about 150 nm is formed all over by a CVD (chemical vapor deposition) method, and then doped with an impurity such as phosphorus. Afterward, the thus doped polysilicon film is patterned into a desired shape by a lithography technique and a dry etching technique to form a gate electrode 504. Next, a silicon oxide film is deposited all over the surface of the substrate by the CVD method, followed by anisotropic dry etching, to form a spacer 505 on the side of the gate electrode 504. Next, an impurity such as arsenic or boron is injected into the silicon oxide film, and a heat treatment is then carried out at 800 to 1000.degree. C. to form a diffusion layer 506. Here, in the case that the MOS transistor is an n-channel type, the diffusion layer containing arsenic is formed, or in the case that the MOS transistor is a p-channel type, the diffusion layer containing boron is formed. The thus formed diffusion layer becomes a source drain region of the transistor.
Next, as shown in FIG. 5(b), a titanium film 507 having a thickness of about 50 nm is formed all over by a sputtering process or the like. successively, a heat treatment is carried out for a period of 30 to 60 seconds under atmospheric pressure in a nitrogen atmosphere. Here, the heat treatment under the nitrogen atmosphere plays a role for forming a self aligned structure of titanium silicide. By the heat treatment under the nitrogen atmosphere after the formation of the titanium film 507, the silicide formation and the nitriding of titanium occur simultaneously, but for titanium on the oxide film, the nitriding alone proceeds and the silicide formation does not proceed, whereby the self aligned structure of titanium silicide can be realized. As a heat treatment apparatus, a lamp annealing apparatus is usually used, and a heat treatment temperature is set in the range of 600 to 650.degree. C. In this way, the silicidation of titanium can be carried out. On the exposed surface of the gate electrode 504 and the surface of the diffusion layer 506, a C49 structure silicide layer 509 of a crystalline structure having a high electric resistivity of about 60 .mu..OMEGA..multidot.cm and titanium nitride 508 are formed, as shown in FIG. 5(c).
Next, as shown in FIG. 6(a), the above-mentioned titanium nitride layer 508 is removed with a chemical agent solution obtained by mixing an aqueous ammonia solution, pure water and a hydrogen peroxide liquid. By passing through the above-mentioned steps, the C49 structure silicide layer 509 can be formed only on the gate electrode 504 and the diffusion layer 506 which defines the source-drain region. Furthermore, a second heat treatment is carried out under atmospheric pressure in a nitrogen atmosphere for about 60 seconds. Here, as a heat treatment apparatus, the above-mentioned lamp annealing apparatus is used, and a treatment temperature is set to about 850.degree. C. By this treatment, the above-mentioned C49 structure silicide layer 509 changes into a C54 structure silicide layer 510 of a crystalline structure having a low electric resistivity of about 20 .mu..OMEGA..multidot.cm, as shown in FIG. 6(b).
In the case that a silicidation technique is applied to the MOS transistor, it is required that the diffusion of the impurity for the formation of the diffusion layer is restrained and a short channel effect of the transistor is restrained in compliance with the high integration of the semiconductor device. As a result, when a junction face of the diffusion layer comes in contact with the silicide region layer, a crystal defect leak current increases, so that the switch operation of the transistor is poor. Therefore, with the formation of the thin junction of the diffusion layer, the formation of the thin silicide film is essential.
However, when the formation of the thin silicide film is attempted, it is difficult to form such a thin silicide film by a conventional technique. This is based on a fact that, in the heat treatment in the nitrogen atmosphere after the formation of the titanium film 507, a reaction rate of the nitriding is higher than that of the silicide reaction, and so, in the case that the thin titanium layer is used, the whole titanium filth is nitrided and the silicide film is not formed. In particular, in the case that arsenic impurity is contained in silicon, the silicide reaction rate is further low, and the reaction rate of the titanium nitride formation relatively increases. In consequence, the thickness of the silicide film extremely decreases.
The above-mentioned phenomenon will be described in more detail. FIG. 7 shows the dependence of a silicide formation rate on an impurity concentration. From the drawing, it is apparent that as the concentration of the impurity injected into silicon is high, the formation rate of the silicide is low. It can be presumed that this phenomenon is due to a fact that, as the concentration of the impurity is high, a monosilicide is stabilized, so that a time taken for the change of the monosilicide into a disilicide in the silicide reaction (a latent time) is prolonged. On the other hand, if the heat treatment for the silicide reaction is done under the nitrogen atmosphere, the nitriding reaction of titanium takes place together with the silicide reaction. The dependence of this nitriding reaction cf titanium on the impurity concentration is shown in FIG. 8. As is apparent from the drawing, the dependence of the nitriding reaction on the impurity concentration in silicon is low, because the nitriding reaction occurs due to the diffusion of nitrogen in titanium. As understood from the above, when the impurity is injected at a high concentration, the nitriding reaction of titanium is relatively faster than the silicide formation reaction. For example, in a silicon board in which the impurity is injected at a high concentration of about 5.times.10.sup.15 ions/cm.sup.2 into the thin titanium film having a thickness of 20 nm or less, the silicide reaction scarcely proceeds and hence the nitriding reaction alone of titanium takes place. As understood from this fact, there has been a problem that the lower limit of the thickness of the titanium film which permits the formation of the thin silicide film naturally depends on the concentration of the impurity.